It's not that BP was all that good and had no drawbacks, it's that XP is linear, grindy and boring as fuck.
With XP you will end up hitting trees and picking up stuff with the sole purpose of getting that 0.01 xp. Most likely you will end up doing so in semi-afk mode (bind z +attack;+duck).
With XP changing server mid-wipe become a very frustrating experience. You need to grind XP without any chance to respond or protect yourself against already leveled players. Sure, you can find some top-level gear but it's a drop in the sea giving the amount of PVP in this game. So, if you joined server mid-wipe you will be a naked playing against top-geared players and there is no way around it except of spending a lot of time grinding XP.
With XP your experience is very linear. You get item X after grinding amount of XP Y. And until you do Z amounts of actions granting you 0.01 XP you won't get item X. Sure, with BP you could hit a barrel and get C4 right away... and run around without top-end weapons and armour for few following weeks. So, you could get AK day one and no body armour for the following weeks. Or you could get your AK after killing and looting raiders at your base. Or buy rushing into middle of someone elses fight and looting somebody. Or by raiding somebody and finding gear inside. Etc.
There is probably more, but nothing really comes into mind. Except that, quite often people are using argument that large groups could get high-end gear day one fast with BP and therefore XP is better. But in reality, large groups will always have few dedicated players that will grind to level 25 in a day or two. So, basically, the same shit is happening but the game become boring and repetitive.